1. Field of the Invention
The present invention relates to an exhaust gas purification apparatus of an engine (referred to hereunder as “exhaust gas purification apparatus”), which reduces the nitrogen oxide (NOx) in the exhaust gas using a liquid reducing agent. In particular, it relates to technology for preventing a failure in the reduction and purification of NOx.
2. Description of the Related Art
As an exhaust gas purification apparatus for removing NOx contained in engine exhaust gas, a type disclosed in Japanese Unexamined Patent Publication No. 2000-27627 has been proposed. In such an exhaust gas purification apparatus, a NOx reduction catalyst is arranged in an engine exhaust gas passage in order to convert NOx into harmless nitrogen (N2) and oxygen (O2) in an oxygenated atmosphere filled with excessive amount of oxygen. Furthermore, a construction is adopted in which an amount of liquid reducing agent as required according to the engine operating state, is supplied by injection from a nozzle into the exhaust gas upstream of the NOx reduction catalyst, in order to enhance the efficiency of NOx purification in the NOx reduction catalyst.
In such an exhaust gas purification apparatus, when the nozzle is at a high temperature due to the exhaust heat, and the injection supply of the liquid reducing agent is stopped, there is a possibility that water content from the liquid reducing agent remaining inside the nozzle evaporates, and reducing agent constituents are deposited and become attached to the inside of the nozzle, thus causing it to become clogged. If clogging occurs in the nozzle, then even though injection supply of the liquid reducing agent is restarted afterwards, there is concern that liquid reducing agent of a necessary amount corresponding to the engine operating conditions is not supplied to the NOx reduction catalyst, and the NOx in the exhaust is exhausted into the atmosphere without being sufficiently reduced and purified.